The aim of this study is to investigate the metabolism of ursodeoxycholic acid (3 alpha, 7 beta-dihydroxycholanoic acid) in man. Two types of studies are planned: In one group of subjects, ursodeoxycholic acid will be administered and with tracer techniques, we will measure daily absorption, pool size, effect on biliary lithogenicity and conversion to chenodeoxycholic acid (3 alpha, 7 alpha-dihydroxyycholanoic acid). In a second of individuals, chenodeoxycholic acid will be given and with tracer technique using (24-C14) ursodeoxycholic acid, the daily production of ursodeoxycholic acid and pool size will be measured. These studies will also provide information on the comparative effect of chenodeoxycholic acid and ursodeoxycholic acid on bile lithogenicity and hepatic function as well as the degree of bacterial metabolism of the respective bile acids.